Bifunctional electrode for an electrochemical cell or a supercapacitor and a method of producing it

ABSTRACT

The present invention concerns an electrode for an electric cell or supercapacitor containing a non-aqueous liquid electrolyte, the electrode comprising an electronically conducting porous first layer including at least one first face covered with a microporous second layer constituted by a polymeric material, the electrode being characterized in that said second layer is produced by coagulation of said polymer from a solution of said polymer impregnating said first face.

The present invention relates to a bifunctional electrode for anelectrochemical cell or a supercapacitor. It also relates to a method ofproducing it.

The electrodes in conventional supercapacitors and electrochemical cellsare separated by a layer of porous insulating material which isimpregnated with electrolyte. That separator constitutes a separatecomponent, which is usually commercially available, which is assembledwith the other components during assembly of the electrochemical element(cell or supercapacitor). It must therefore have sufficient intrinsicmechanical strength for it to be capable of being manipulated and ofresisting the stresses of automated industrial production. Thosecomponents have the problem of being expensive and not specificallyadapted to the type of cell in which they are used. Conventionalseparators limit improvements in the performance of electric cells andsupercapacitors.

In order to facilitate assembly of the components, a separator supportedby an electrode has been proposed. The porous sheet acting as aseparator may be manufactured initially, and it is then applied to theelectrode by mechanical means such as rolling or coextrusion with themetal of the electrode (European patents EP-0 219 190 and EP-0 243 653),or by a chemical method, such as using an adhesive (EP-0 195 684). Theoperation is difficult to perform without damaging the separator, whichmeans a thick, mechanically strong separator must be used. Further, theadhesion obtained is imperfect and the risk of delamination is high ifthe dimensions of the electrode vary.

The separator can also be formed directly on the electrode which acts asa support.

As an example, one method consists of depositing, on the electrode,separator material in the form of a continuous layer which is renderedporous during a subsequent operation, normally by means of apore-forming additive added to the material. European patentapplications EP-0 143 566 and EP-0 146 246 describe a metallic lithiumelectrode on which a non-porous layer of a protective material isdeposited, for example by rolling or extrusion. That material is thenrendered porous by reaction with one of the electrode components or bythe presence of an additive. That method involves several operations andthe use of pore-forming additives whose choice is limited to those whichare completely eliminated or to those which are compatible with thecomponents and operating conditions of the electrochemical element.

Other methods deposit a porous layer directly onto a solid metalelectrode. European patent application EP-0 143 562 describes thedeposition of a porous layer on a lithium electrode from a solutionwhich is then evaporated off. In other methods, deposition of a monomerin solution can be followed by polymerization in situ.

U.S. Pat. Nos. 4,885,007 and 4,524,509 describe a method of envelopinglead electrodes in a microporous separator. That method consists ofimmersing the electrodes in a coating solution containing a polymer anda filler, then drying in air. The set of operations is repeated a numberof times to obtain a separator with sufficient thickness. That methodrequires a number of manipulations.

U.S. Pat. No. 3,023,261 describes a method of producing anelectrode-separator assembly for secondary cells containing an aqueousalkaline electrolyte. The electrodes are immersed in a solutioncontaining a synthetic resin which is insoluble in water and a polymerwhich swells in water, then washing with water to eliminate any tracesof solvent. The polymer which swells in water is intended to give theelectrolyte access to the electrode. It is advisable to use the maximumquantity of that polymer without compromising the mechanical strength ofthe separator. The characteristics of the separator obtained essentiallydepend on the choice and concentration of polymer which swells in water.

Known methods of forming the separator directly on the electrode canimprove adhesion between the separator and the electrode. However, thosemethods include a number of steps and often use forming additives whichmay have an undesirable effect on the operation of the electrochemicalelement.

An object of the present invention is to provide an electrode which alsoacts as a separator which is simpler and easier to manufacture thanusing known methods.

In one aspect, the present invention provides an electrode for anelectrochemical cell or supercapacitor containing a non-aqueous liquidelectrolyte, the electrode comprising an electronically conductingporous first layer including at least one first face covered by a secondmicroporous layer constituted by a polymeric material, the electrodebeing characterized in that said second layer is produced by coagulationof said polymer from a solution of said polymer impregnating said firstface.

The electrode of the invention thus simultaneously acts as the seat ofthe electrochemical reaction which uses the active material contained inthe first layer, and as the separator formed by the microporous secondlayer which is impregnated with the liquid electrolyte introduced intothe cell or supercapacitor. The first and second layers are thusintimately connected to each other and they can follow any dimensionalvariations in the electrode during cycling.

Preferably, the pore volume of the second layer is in the range 30% to95%. A porosity of more than 95% affects the mechanical strength of theseparator, while if the porosity is insufficient, then the second layerintroduces too high a series resistance.

Preferably, the thickness of the second layer is in the range 10 μm to100 μm. If the second layer is too thin, the material distribution willbe non-uniform and the mechanical strength of the separator will beinsufficient (perforation risk). Increasing the thickness of the secondlayer beyond 100 μm does not improve cell performance, but does reducethe capacity per unit volume by increasing the volume occupied by theseparator.

The polymers constituting the second layer are selected for theirability to withstand operating conditions and their chemical inertnessto the components of the electrochemical cell or supercapacitor.

In a first variation, the second layer is constituted by a polymericmaterial selected from poly(vinylidene fluoride) (PVDF), poly(vinylchloride) (PVC), poly(methyl methacrylate), cellulose triacetate (CA), apolysulfone, a polyether, a polyolefin such as polyethylene (PE),poly(ethylene oxide) (PEO), polypropylene (PP) and copolymers thereof.

In a second variation, the second layer is constituted by a polymericmaterial which is an alloy of poly(vinylidene fluoride) PVDF (CH₂ --CF₂)with a polymer selected from a polysulfone, poly(methyl methacrylate),poly(vinylpyrrolidone), and copolymers of poly(vinylidene fluoride) andpoly(tetrafluoroethylene) (PTFE), of poly(vinylidene fluoride) andpropylene hexafluoride, or of poly(vinyl acetate) (PVAC) and poly(vinylalcohol) (PVA). PVDF has high mechanical strength and goodelectrochemical characteristics, and its alloys have good fatigue andabrasion resistance.

In some cases, it is necessary to add a wetting agent to the secondlayer in an amount of less than 10% by weight of polymer. This agent canimprove penetration and distribution of the electrolyte in theseparator.

In an embodiment of the invention, the second layer is constituted by across-linked polymeric material. The structure of the polymer is thusrendered more rigid, and when the material is impregnated withelectrolyte, swelling remains low.

The first layer of the electrode is formed in known fashion depending onthe type of cell or supercapacitor to be produced.

When the mechanical strength of the first layer is insufficient, or whenthe first layer is not sufficiently conductive, the electrode of theinvention also includes a current conducting support. This support maybe constituted, for example, by a metal foil, an expanded metal, or ametal grid or fabric.

In a first embodiment of the invention, the support is in contact withthe first face of the first layer and the second layer adheres to thesecond face of the first layer. The electrode is a stack composed of thesupport, the first layer containing the active material, and the secondpolymeric layer.

In a second embodiment of the invention, the support is included in thefirst layer and the second layer adheres to the first face and thesecond face of the first layer. For example, the support can be placedin the center of the first layer and each of the two faces of the firstlayer is covered with the second polymeric layer.

In a further aspect, the present invention provides a supercapacitorincluding an electrode in which the first layer contains anelectrochemically active material selected from activated charcoal andan oxide of a transition metal oxide such as iridium or ruthenium, and asecond layer constituted by poly(vinylidene fluoride).

An electrode of this type is particularly suitable for a supercapacitorcontaining an organic electrolyte. The supercapacitor includes at leastone electrode in accordance with the invention but it may also includeboth an electrode and a counter-electrode which are in accordance withthe invention.

In a still further aspect, the invention provides an electrochemicalcell comprising an electrode in which the first layer contains anelectrochemically active material selected from materials which canintercalate an alkaline cation, and a second layer constituted bypoly(vinylidene fluoride). An electrode of this type is particularlyintended for use in a lithium electrochemical cell. The electric cellcan include an electrode (anode or cathode) in accordance with theinvention, and a conventional counter-electrode; it also contains anon-aqueous electrolyte composed of a lithium salt in an organicsolvent. The electrochemical cell can also include both a cathode and ananode in accordance with the invention placed face to face, such thatthe second layer of each electrode is in contact with that of the other.In which case, the thickness of the second layer in each electrode isadjusted accordingly.

For a lithium electrochemical cell, the material which can intercalate alithium ion is, for example, graphite or coke for an anode, or for acathode, a lithiated or non lithiated oxide of a transition metal suchas manganese, vanadium, nickel, cobalt, chromium, or titanium.

The use of an electrode in accordance with the present invention meansthat the separator as a distinct component can be omitted while theionic percolation in and mechanical strength of the electric cell areimproved. The second layer can have a high porosity and thus retain asufficient quantity of electrolyte to ensure a maximum lifetime for thecell or the supercapacitor. An electrode of this type can be used withthe majority of non-aqueous electrolyte solvents.

The electrode of the present invention can increase the reliability ofcells or supercapacitors containing it, and improve their performanceper unit volume and per unit mass.

The present invention also provides a method of producing an electrodefor an electrochemical cell or a supercapacitor containing a non-aqueousliquid electrolyte, comprising a first layer containing theelectrochemically active material and a microporous second layerconstituted by a polymeric material. The method comprises the followingsteps:

producing the first layer;

coating the first layer with a film of a solution of the polymer in avolatile solvent;

then bringing the film into contact with a first volatile non-solventwhich is miscible with the solvent;

finally, drying the electrode to eliminate the solvent and thenon-solvent.

The solvent is an organic solvent selected from cyclohexanone,dichloromethane, dimethylacetamide (DMA), dimethylformamide (DMF),hexamethylphosphoramide (HMP), dimethylsulfoxide (DMSO),triethylphosphate (TEP), 1-methyl-2-pyrrolidone or N-methylpyrrolidone(NMP), and mixtures thereof. Preferably, an organic solvent is used inwhich the polymer dissolves without difficulty and which can readily beeliminated by heating at a moderate temperature without risking damageto the active material.

The selected polymer is taken up in a concentrated solution in thesolvent. The concentration of polymer must not be too high since that isone of the parameters which determines the porosity of the second layer;preferably, the solution contains at least 50% of solvent.

In a variation, the solution also contains a non-solvent in a proportionwhich is insufficient to cause precipitation of the polymer. The term"non-solvent" means a liquid in which the polymer is not soluble (strongnon-solvent) or is only very slightly soluble (weak non-solvent) at theoperating temperature. When the non-solvent selected is pure water or amixture including it, this temperature is in the range 5° C. to 80° C.The presence of a small quantity of a weak non-solvent encourages thethree-dimensional organization of the polymer during precipitation.

The dissolved polymer is deposited on the surface of the first layer ofthe electrode using a known method such as immersion, coating, spraycoating, etc. . . . This surface contains irregularities and a certainporosity which are filled in by the solution and which facilitatebonding of the second layer.

In a variation of the method of the invention, the first layer issuperficially impregnated with a wetting agent before being covered withthe film of dissolved polymer. The wetting agent is, for example, avolatile organic solvent.

The film of dissolved polymer is then placed in contact with thenon-solvent. The solvent is then replaced by the non-solvent with whichit is miscible, causing precipitation of the polymer. Subsequentrecovery of the solvent extracted by the non-solvent is facilitated. Ifthe selected non-solvent is water, the method of the invention has theadvantage of not polluting the environment and facilitating recycling ofthe solvents.

The first and/or second non-solvent is/are selected from water, ethanol(CH₃ OH), ethylene glycol, glycerol, acetone, propylene carbonate (PC),dichloromethane, ethyl acetate, butanol, pentanol, acetonitrile, andmixtures thereof.

A film of solid polymer thus covers the surface of the electrode. It issufficient to evaporate the non-solvent, and possibly a portion of theresidual solvent, by moderate heating to obtain the electrode of theinvention. The steps of the method can be repeated several times if athicker second layer is desired.

In a further variation, the method also includes a step forcross-linking the polymer after drying the electrode, the solutioncontaining a cross-linking agent. This supplemental step is necessaryfor some applications where the separator must have greater mechanicalstrength.

The method of producing an electrode of the invention comprises simpleoperations which allow continuous production of all the constituents ofthe electric cell. By omitting the step of cutting out a separator as acomponent, the loss of material which normally occurs because of cuttingscrap is avoided, and thus the quantity of lost material is minimized.As a result, the cost of producing the electrode of the presentinvention is lower than that of a prior art electrode associated with aconventional separator. By limiting the number of operations, anelectrode-separator assembly is obtained which is simpler and morereliable to produce than by using known methods.

Other features and advantages of the present invention become apparenton reading the following examples of embodiments which, of course, aregiven by way of illustration and are in no way limiting, described withreference to the accompanying drawings in which:

FIG. 1 is an exploded sectional view of a button type lithium cell ofthe prior art;

FIG. 2 is a partial schematic view of an electrode in accordance withthe present invention;

FIG. 3 is analogous to FIG. 1 for a lithium cell comprising an electrodein accordance with the present invention;

FIG. 4 is a graph with curves showing variation in the capacity C (inmAh/gram (g)) discharged by a cell of the invention and by a prior artcell as a function of number of cycles during cycling;

FIG. 5 is a graph with a curve showing variation in the capacitance F(in Farads) discharged by a supercapacitor of the invention duringcycling;

FIG. 6 is an electron micrograph (0.5 cm=1 μm) showing the surface ofthe first layer of the electrode;

FIG. 7 is an electron micrograph (1 cm=1 μm) showing the surface of thesecond layer of the electrode, in accordance with the invention.

EXAMPLE 1

In order to test a prior art electrode, a button type lithium cell 1 asshown in FIG. 1 was produced.

The electrolyte was a solution of a mixture of lithium salts in anon-aqueous solvent. The non-aqueous solvent was composed of 20% ofpropylene carbonate (PC), 20% of ethylene carbonate (EC) and 60% ofdimethyl carbonate (DMC), in which was dissolved a mixture of lithiumsalts composed of lithium trifluoromethanesulfonimide LiN(CF₃ SO₂)₂(LiTFSI) at a concentration of 1.5M and lithium perchlorate LiClO₄ at aconcentration of 0.1M.

Positive electrode 2 was composed of a current collector on which alayer constituted by 90% by weight of graphite mixed with a PVDF polymerbinder had been deposited. The electrode was dried at 110° C. thenimpregnated with electrolyte and hot rolled before being positioned inthe cell can 3.

Separator 4 was constituted by a polypropylene sheet with 45% porosity,a thickness of 50 μm and diameter 22 mm, with the trade name "CELGARD2502". The separator was impregnated with electrolyte and positioned onpositive electrode 2.

Negative electrode 5 was a 14 mm diameter pellet of lithium metal. Astainless steel spacer 6 ensured current pick-up and a spring 7 kept thedifferent elements of the cell in contact. The cell was covered by a cap8 and gasket 9 ensured that the cell was sealed.

Cell 1 was cycled between 0 and 2 volts at a current of 20 mA/g ofgraphite. FIG. 4 shows the discharge curve 42 of prior art cell 1. Theinitial capacity was 240 mAh/g of graphite. After 25 cycles, it was nomore than 150 mAh/g, meaning a reduction in capacity of 37.5%.

EXAMPLE 2

In order to test an electrode in accordance with the present invention,a button type electric cell 31 was produced as shown in the explodedview of FIG. 3, comprising an electrode 32 in accordance with theinvention.

FIG. 2 is a fragmentary section view of portion II of electrode 32.Electrode 32 was composed of a current collector of copper foil; on oneof its two faces, a first layer 24 had been deposited from a pasteconstituted by at least 90% by weight of graphite mixed with 10% of aPVDF polymer binder. The electrode was dried at 110° C. then hot rolled.Its surface can be seen in FIG. 6.

Production of the second layer 25 of electrode 32 comprised thefollowing steps. The face of first layer 24 opposite to currentcollector 22 was coated with a solution containing 12.5% by weight ofPVDF and 87.5% of TEP to deposit a film of solution on its surface.After draining, the electrode was immersed in water for 20 minutes. Thepolymer was caused to precipitate since the water was a strongnon-solvent. The electrode was then dried in air, firstly at 35° C. thenat 120° C., to remove all traces of water. A solid PVDF layer ofthickness 50 μm and porosity 75% was obtained, which adhered strongly tothe first layer. Its resistivity was about five times lower than that ofthe separator described in Example 1. Its surface can be seen in FIG. 7.

The prepared electrode was impregnated with an electrolyte with the samecomposition as that described in Example 1 to cause swelling of thesecond layer. It was positioned in cup 3 so that current collector 23was in contact with cup 3. A lithium metal anode 5 was placed above it,directly in contact with the second layer 25. A stainless steel spacer 6ensured current pick-up and spring 7 kept the different elements of thecell in contact. The assembly was covered with a cap 8, and gasket 9ensured the cell was sealed.

Cell 31 was cycled between 0 and 2 volts at a current of 20 mA/g ofgraphite. FIG. 4 shows the discharge curve 41 of cell 31 of theinvention. The initial capacity was 350 mAh/g of graphite. After 50cycles, it was 300 mAh/g, i.e., a reduction limited to 14.3%. Initially,the capacity of cell 31 of the invention was greater by 46% than that ofthe prior art cell 1 described in Example 1, and this value wasmaintained over more than 50 cycles.

EXAMPLE 3

In order to test an electrode of the present invention, an electric cellwhich was analogous to that of Example 2 was produced, but in which thesecond layer of the positive electrode was produced as follows.

The face of the first layer was coated with a solution of 9.1% by weightof PVDF, 54.5% of NMP and 36.4% of ethanol to deposit a film of solutionon its surface. After draining, the electrode was immersed in water at80° C., and then dried in air at 35° C. A solid PVDF layer with aporosity of 25% was obtained.

EXAMPLE 4

In order to test an electrode of the present invention, a button typesupercapacitor was produced which includes an electrode of theinvention. The electrode was composed of a current collector which wasan aluminum sheet of thickness 20 μm. One of its two faces was coated,using a blade, with a paste constituted by 80% by weight of activatedcharcoal with the trade name "SX ULTA NORIT" mixed with 20% of a PVPpolymer binder in aqueous solution. The electrode was dried for one hourat 110° C. then hot rolled. The surface of the first layer was thenimpregnated with a wetting agent composed of a mixture of equal volumesof ethanol and PC.

This also meant that air bubbles could be readily eliminated and PVDFpenetration into the electrode could be limited.

Production of the second layer involved the following operations. Aplate whirler (spinning disk) was used to spread a solution containing9.5% by weight of PVDF, 40% of NMP and 50.5% of PC as a weak non-solventon the surface of the first layer. The electrode was immersed in waterfor 15 to 20 minutes to cause precipitation of the polymer. Theelectrode was dried in air at 35° C. then 110° C. to eliminate alltraces of water and solvents. A layer of PVDF of 50 μm thickness and 50%porosity was obtained, which could not be separated from the firstlayer.

The prepared electrode was combined with an analogous counter-electrodewithout the second layer. The electrolyte was a mixture of PC andethylammonium tetrafluoroborate Et₄ NBF₄ at a concentration of 1M

The supercapacitor was discharged several times at a current of 1 mA/gof activated charcoal. FIG. 5 shows how the capacitance (curve 51) ofthe supercapacitor of the invention varied during cycling. Excellentstability of capacitance can be seen over more than 1000 cycles.

EXAMPLE 5

In order to test an electrode of the present invention, a button typesupercapacitor analogous to that of Example 3 was assembled, however itincluded two electrodes of the invention placed face to face so that thesecond layers of the two electrodes were in contact. In this case, thethickness of the second layer of each electrode was 20 μm. The presenceof the second layer improved the mechanical strength of each of theelectrodes. Further, the quantity of polymer used could be minimizedcompared with the case where the separator was carried by one electrodeonly. In this case the total thickness of the second layers was only 40μm.

The present invention is not limited to the embodiments described andshown, but can be varied in a number of ways which are available to theskilled person without departing from the spirit of the invention. Inparticular, without going beyond the ambit of the invention, the firstlayer could be produced from any known electrochemically activeelectrode material and any normal polymeric binder. The electrode of theinvention can be assembled in electrochemical elements of variousgeometric shapes and dimensions: cylindrical, prismatic, etc. . . .

We claim:
 1. An electrochemical cell or supercapacitor containing anon-aqueous liquid electrolyte and an electrode, the electrodecomprising an electrically conductive porous layer having at least oneface covered by a microporous, coagulated polymeric layer, wherein saidpolymeric layer is intimately connected to said face and is produced byprecipitation of said polymer using a non-solvent from a solution ofsaid polymer impregnating said face.
 2. An electrochemical cell orsupercapacitor according to claim 1, in which the microporous,coagulated polymeric layer has a pore volume in the range 30% to 95%. 3.An electrochemical cell or supercapacitor according to claim 1, in whichthe microporous, coagulated polymeric layer has a thickness in the range10 μm to 100 μm.
 4. An electrochemical cell or supercapacitor accordingto claim 1, in which said microporous, coagulated polymeric layercomprises a polymeric material selected from poly(vinylidene fluoride),poly(vinyl chloride), poly(methyl methacrylate), cellulose acetate, apolysulfone, a polyether, or a polyolefin, and copolymers thereof.
 5. Anelectrochemical cell or supercapacitor according to claim 1, in whichsaid microporous, coagulated polymeric layer comprises a polymericmaterial which is an alloy of poly(vinylidene fluoride) with a polymerselected from a polysulfone, poly(methyl methacrylate) andpoly(vinylpyrrolidone), and copolymers of poly(vinylidene fluoride) andpoly(tetrafluoroethylene), of poly(vinylidene fluoride) and propylenehexafluoride, or of vinyl acetate and vinyl alcohol.
 6. Anelectrochemical cell or supercapacitor according to claim 1, in whichsaid microporous, coagulated polymeric layer also contains a wettingagent in a proportion of less than 10% by weight of said polymer.
 7. Anelectrochemical cell or supercapacitor according to claim 1, in whichsaid microporous, coagulated polymeric layer comprises a cross-linkedpolymeric material.
 8. An electrochemical cell or supercapacitoraccording to claim 1, further containing a current conducting support.9. An electrochemical cell or supercapacitor according to claim 8, inwhich said support is positioned in contact with said at least one faceof said electrically conductive porous layer and said microporous,coagulated polymeric layer adheres to a second face of said electricallyconductive porous layer.
 10. An electrochemical cell or supercapacitoraccording to claim 8, in which said support is included in saidelectrically conductive porous layer and said microporous, coagulatedpolymeric layer adheres to both faces of said electrically conductiveporous layer.
 11. A supercapacitor including an electrode in accordancewith claim 1, in which said electrically conductive porous layercontains an electrochemically active material selected from activatedcharcoal and a transition metal oxide, and said microporous, coagulatedpolymeric layer is constituted by poly(vinylidene fluoride).
 12. Anelectrochemical cell including an electrode in accordance with claim 1,in which said electrically conductive porous layer contains anelectrochemically active material selected from materials which canintercalate an alkaline cation, and said microporous, coagulatedpolymeric layer is constituted by a poly(vinylidene fluoride) polymericmaterial.
 13. A method of producing an electrode, comprising thefollowing steps:(a) providing an electrically conductive porous layer;(b) coating said electrically conductive porous layer with a film of asolution of a polymer in a volatile solvent, the polymer is selectedfrom:(A) poly(vinylidene fluoride), poly(vinyl chloride), poly(methylmethacrylate), cellulose acetate, a polysulfone, a polyether, apolyolefin, and copolymers thereof; or (B) an alloy of poly(vinylidenefluoride) with a polymer selected from a polysulfone, poly(methylmethacrylate) and poly(vinylpyrrolidone), and copolymers ofpoly(vinylidene fluoride) and poly(tetrafluoroethylene), ofpoly(vinylidene fluoride) and propylene hexafluoride, or of vinylacetate and vinyl alcohol; (c) after coating step (b), bringing saidfilm into contact with a volatile non-solvent which is miscible withsaid solvent, whereby an electrode is formed; (d) drying said electrodeof step (c) to eliminate said solvent and said non-solvent.
 14. A methodof producing an electrode according to claim 13, in which said solventis an organic solvent selected from cyclohexanone, dichloromethane,dimethylacetamide, dimethylformamide, hexamethylphosphoramide,dimethylsulfoxide, triethylphosphate, N-methylpyrrolidone, and mixturesthereof.
 15. A method of producing an electrode according to claim 13,in which said solution also comprises a non-solvent in a proportionwhich is insufficient to cause precipitation of the polymer.
 16. Amethod of producing an electrode according to claim 13, in which saidnon-solvent is selected from water, ethanol, ethylene glycol, glycerol,acetone, propylene carbonate, dichloromethane, ethyl acetate, butanol,pentanol, acetonitrile, and mixtures thereof.
 17. A method of producingan electrode according to claim 13, in which said electricallyconductive porous layer is superficially impregnated with a wettingagent before being covered with said film of said solution of saidpolymer.
 18. A method of producing an electrode according to claim 13,further comprising a step for cross-linking said polymer after dryingsaid electrode, said solution containing a cross-linking agent.